1. Field of the Invention
The invention relates to a hand-off control technique, particularly to a hand-off control method which is ready for a plurality of communication systems and a wireless communication apparatus using the method.
2. Description of the Related Art
Recently, cdma2000 1×EV-DO (hereinafter referred to as “EV-DO”) has been developed as a next-generation high-rate radio communication system. The EV-DO is a version of specializing cdma2000 1× (hereinafter also referred to as “1×”) provided by expanding cdma2000 to the third-generation system for data communications and increasing the transmission rate. “EV” means Evolution, and “DO” means Data Optimized.
The EV-DO system is approximately the same as the cdma2000 1× system in the configuration of a radio interface of a reverse channel from a radio communication terminal to a base station. As for the configuration of the radio interface of a forward channel from a base station to a radio communication terminal, whereas the bandwidth of 1.23 MHz is the same with that of the cdma2000 1× system, the modulation method and the multiplexing method are much different from those of the cdma2000 1× system. As for the modulation method, whereas QPSK and HPSK are used in the cdma2000 1× system, QPSK, 8-PSK, or 16QAM is selectively used in the EV-DO system according to a reception state of a forward channel of a radio communication terminal. As a result, a high transmission rate with low error correction capability is used when the reception state is good, and a low transmission rate with high error correction capability is used when the reception state is bad.
As for the multiplexing method for enabling multiple communications from one base station to a plurality of radio communication terminals, TDMA (Time Division Multiple Access) is employed in which communications with a plurality of radio communication terminals are performed in such a manner that time is divided in units of 1/600 second, a communication is performed with one radio communication terminal in each unit time, and the radio communication terminal to be communicated is switched every unit time, rather than CDMA (Code Division Multiple Access) which is employed in the cdmaOne system and the cdma2000 1× system.
A radio communication terminal measures a carrier-to-interference power ratio (hereinafter abbreviated as CIR) of a pilot signal as an index of a reception state of a forward channel from a base station to be communicated, predicts a reception state during the next reception time slot on the basis of a variation of the CIR, and notify “a maximum transmission rate which enables to receive with a error ratio that is lower than a predetermined rate”, which is expected from the predicted reception state to the base station as data rate control bits (hereinafter referred to as DRC) predetermined. The predetermined error rate is usually set to about 1% though it depends on the system design. The base station receives the DRCs from a plurality of radio communication terminals, and a scheduler function in the base station determines with which radio communication terminal is to communicate in each division unit time. Basically, as high a transmission rate as possible is decided on the basis of a DRC sent from each radio communication terminal and is used for a communication with it.
With the above configuration, the EV-DO system enables a maximum transmission rate of 2.4 Mbps (mega-bits per second) per sector in a forward channel. This transmission rate is the sum of amounts of data communications from one base station to a plurality of radio communication terminals in one frequency band and in one of a plurality of sectors (usually, a plurality of sectors exist). The transmission rate increases if a plurality of frequency bands are used.
JP-A-2002-300644 is referred to as a related art.
If a wireless communication terminal uses an RF circuit and an antenna to be in conformity with both of the EV-DO system and the 1× system, the user does not have to carry two terminals for the EV-DO system and the 1× system, thereby improving convenience. Since the wireless communication terminal switches the RF circuit and the antenna according to the situation in order to be in conformity with the two communication systems, the wireless communication terminal can detect an incoming call of the 1× system during data communication of the EV-DO system. However, in the case where a hand-off of the EV-DO system occurs during data communication of the EV-DO system, and further an incoming call monitoring time of 1× system starts before completion of the hand-off processing of the EV-DO system, the wireless communication terminal halts the hand-off processing of the EV-DO system to execute the incoming call monitoring processing of the 1× system. As a result, a monitoring timer of the hand-off is terminated in the base station of the EV-DO system so that communication of the EV-DO system is cut off.